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 TM 9-8000
downward stroke is also an intake and an exhaust stroke.
center, the spark plug ignites the fuel and air mixture,
As the piston moves from bottom dead center back to top
beginning the downward power stroke again.
dead center, it is going through a compression stroke.
c. The Fuel and Lubrication System. The fuel and
a. Downward Stroke (A, Fig. 2-14). The piston
air mixture must first pass through the crankcase before
begins the power stroke at top dead center. As the
it gets to the combustion chamber. For this reason, the
exploding fuel and air mixture pushes the piston
fuel and air mixture must also provide lubrication for the
downward, it first covers the inlet port. This seals the
rotating and reciprocating parts. This is accomplished by
crankcase.
As the piston continues downward, it
mixing a small percentage of oil with the fuel. The oil,
pressurizes the sealed crankcase, which contains a
mixed with the fuel and air mixture, enters the crankcase
vaporized fuel and air mixture. As the piston continues to
in a vapor that constantly coats the moving parts.
bottom dead center, it uncovers the intake and the
d. Power Output. It may seem like a two-stroke
exhaust ports. The pressure built up in the crankcase
forces the fuel and air mixture into the cylinder through
cycle engine will put out twice as much power as a
the intake port. The top of the piston is shaped to divert
comparable four-stroke cycle engine because there are
the mixture upward and away from the exhaust port. As
twice as many power strokes. However, this is not the
the mixture enters the cylinder, it displaces and pushes
case. Because the force of the fuel and air mixture
the burnt gases out through the exhaust port.
entering the cylinder must be relied upon to get rid of the
burnt gases in the cylinder from the last power stroke,
b. Upward Stroke (B, Fig. 2-14). As the piston
there is a certain amount of dilution of it. The mixing of
moves upward, it covers the intake and exhaust ports.
the intake mixture with exhaust gases reduces its
This seals the upper cylinder so that the upward
potential power output. Also, with the inlet and exhaust
movement of the piston compresses the fuel and air
ports opened together, a certain amount of the fuel and
mixture. At the same time, the upward movement of the
air mixture is lost. There is also a much shorter period
piston creates a suction in the crankcase so that as the
that the inlet port is open. This reduces the amount of
inlet port is uncovered, a mixture of fuel and air is drawn
power from each powerstroke.
into the crankcase. As the piston reaches top dead
Figure 2-14. The Two-Stroke Cycle.
2-9
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